What additional evidence does the author provide that suggests Bruno doesn't believe Shmuel's story?

From the text:

'You can't have all lived in the one room,' said Bruno, shaking his head. 'That doesn't make any sense.'

'All of us,' said Shmuel, nodding his head. 'Eleven in total.'

Bruno opened his mouth to contradict him again - he didn't really believe that eleven people could live in the same room together - but changed his mind.